The present invention relates generally to the investment casting art, and more specifically to a new investment material for use in making refractory molds.
Non-ferrous investment casting molds are prepared by placing a pattern assembly or tree in a flask and then filling the flask with a refractory investment slurry which is allowed to harden in the flask to form the mold. The pattern assembly or tree is comprised of a plurality of patterns having the configurations of the desired metal castings, the patterns being made of wax, plastic or other expendable material. After the investment slurry has set hard, the patterns are melted out of the mold by heating it in an oven, furnace or autoclave. The mold is then fired to an elevated temperature, as for example 1350.degree. F., to remove water and burn off any residual pattern material in the casting cavities. In many instances, the mold is first cooled to a lower temperature in order to obtain optimum casting conditions before filling it with molten metal. For example, it is the practice to cool the molds to a temperature in the range of from room temperature to about 400.degree.-500.degree. F. when casting aluminum.
Conventional non-ferrous investment formulations are comprised of a binder and a refractory made up of a blend of fine and coarse particles. The refractory usually is wholly or at least in part a silica, such as quartz, cristobalite or tridymite. Calcined fire-clay also is often used as a part of the refractory. The binder is typically a fine gypsum powder (calcium sulfate hemihydrate).
The binder and refractory, together with minor chemical additives to control setting or hardening characteristics, are dry blended to produce the investment. The dry investment is then prepared for use by mixing it with sufficient water to form a slurry which can be poured into the flask around the set-up. Vacuuming of the slurry and vibration of the flask are frequently employed steps to eliminate air bubbles and facilitate filling of the flask.
A serious problem encountered with conventional investment molds is the frequent occurrence of cracking during the heating and/or cooling cycles and during the metal casting operation. If a vacuum is applied to the molds during pouring of the metal, the molds are subjected to additional stresses which can contribute to cracking.
Mold cracking results in metal flash on the castings which must be removed by expensive finishing operations. Mold cracking also permits particles or flakes of investment material to break loose and fall into the mold cavities. This can produce inclusions in the castings and cause them to be rejected. In instances where cracking is especially severe, the molten metal can leak through the mold wall so that the entire mold must be scrapped.